DNA is a polymer of nucleotides (Google “nucleotide” for the chemical structure). Each nucleotide contains a phosphate group, a sugar group and a nitrogenous base. It is the “base” that is part of the DNA code. There are four types of nucleotide, containing four types of bases: A, G, C and T. The nucleotides are joined together in a long string-like molecule, one after the other in a chain, like popper beads! So, a “slice” of DNA can be anything from one nucleotide long, to a whole chromosome in length, which would be millions of nucleotides long. The molecule is very very thin (so thin it’s hard to describe) but it is quite long. If we took all the DNA in a single cell (i.e. 46 chromosomes worth) and laid it end to end, it would be 2 metres long.